It is the weeks that get you ready, not the long run

Giovanni Tanda, a researcher at the University of Genoa, gathered the training logs of 22 runners aged 28 to 54 across 46 marathons over five years. He looked at the eight weeks before each race.

How fast they trained predicted their marathon time best. Of the ways he counted how much they ran, the weekly total was the one that predicted it: the more kilometres in an average week, the better the marathon went, and adding it to their training pace made the prediction sharper. The length of their average run and their single biggest week predicted the time poorly.

What trains you is the time you spend under stress, so plan your training in hours, not kilometres. A 45-minute run at base, an effort you could hold for hours, asks about as much of a beginner's heart and lungs as of a strong runner's. The strong runner covers far more ground in those minutes, but both spend the same time under stress. And as you get fitter, the same minutes carry you further. Your weekly distance grows even in weeks when your hours do not.

Around two and a half hours of running is a tipping point. Past it, the recovery cost rises steeply while what you gain from the extra time levels off. One study shows how steep that rise is. When 30 amateur runners ran 10 km, a half marathon or a marathon, researchers measured creatine kinase in their blood, an enzyme that leaks out of damaged muscle. It rose about as much after the half as after 10 km. After the marathon it rose more than four times as much, and three days later it was still that high.

So build your long run up to about two and a half hours and keep it there. Then grow the rest of your week. On the bike the same point comes at about four hours. A long swim stays shorter, at about 80 minutes.

A week that grows fast feels like the risky part of training. The research points somewhere else. Researchers followed 5,205 adult runners for a year and a half and read 588,071 of their runs from the GPS devices they trained with. More than a third of them had an overuse injury along the way.

When a single run was more than 10 percent longer than the runner's longest of the previous 30 days, the injury rate rose by more than half. When a run was more than twice that longest, the rate more than doubled. A week that grew a lot over the week before showed no such link.

A run like that goes past your tissue tolerance, the load your tendons, bones, joints and muscles can take. Your heart and lungs may cope with it easily. Your tendons, bones and joints have not yet carried a run that long.

So keep each long run close to your longest of the last month.

Of everything in your training, your total volume, week after week, matters by far the most. When your weeks are in place, aim them at goals you can reach.